This invention relates to sheet feeding apparatus and more particularly to apparatus for feeding blanks for corrugated cardboard and the like into printing and container fabricating machinery.
In the fabrication of containers of materials such as corrugated cardboard, container blanks are individually fed into fabrication machinery where they may be printed, die cut, folded and/or glued. Such feeders commonly feed the blanks from the bottom of a stack which is bulk loaded on a feeding table and apparatus are provided for sequentially feeding the sheets from the stack into the machine from the bottom of the stack. One type of such feeding mechanism includes a perforated suction head carried on a hollow shuttle. Such shuttles are normally located adjacent to the fabricating machinery nip rolls and below the stack of blanks. The shuttle is commonly cycled back and forth toward the nip rolls while vacuum is initially applied to the interior of the shuttle for gripping the lowermost blank when the shuttle is in its rearmost position and the hollow shuttle interior is vented as it moves toward a forward position for releasing the blank whereupon it may be engaged by the nip rolls for movement into the fabricating machinery.
In prior art apparatus, valves connected the shuttle to a vacuum pump through elongate tubing. This provides a substantial volume that must be evacuated during each shuttle cycle and produces a substantial flow resistance. Further, some prior art suction feeders required filters between the vacuum pump and the shuttle to remove paper scraps and lint which filters also increased flow resistance. As a result of these difficulties, prior suction feeders had difficulties in clamping onto sheets, particularly if the latter were warped.